In the current UMTS (universal mobile telecommunication systems) specifications or the current WCDMA (wideband code division multiplexing access) specifications, a fractional dedicated physical channel (F-DPCH) is introduced in order to increase capacity in a network by reducing code usage in the downlink. This is considered especially important for low bit rate services like e.g. voice-over-IP (VoIP). However, with the introduction of the F-DPCH the signaling radio bearers (SRB) have to be mapped on the high-speed downlink shared channel (HS-DSCH) since there is no room or space for the signaling radio bearers on the F-DPCH.
In addition, downlink signaling messages on the HS-DSCH can only be transmitted from a serving HS-DSCH cell since the HS-DSCH transport channel does not support transmission from several cells i.e. does not support macro diversity. Therefore, in situations wherein fast changing propagation conditions occur e.g. due to challenging radio environment or due to high velocity of a UE, the UE may lose contact with it serving HS-DSCH serving cell before a handover command can reach the UE so that the connection will be lost. Again, this is because in current handover procedures, i.e. the SRB is mapped on the HS-DSCH, a cell change command and all other signaling and data have to be transmitted over the source cell, i.e. the HS-DSCH serving cell. If a loss of the HS-DSCH serving cell occurs, the UE has no quick means of re-accessing the network in an efficient way. The only way of recovery is in conjunction with a procedure where the UE eventually detects the loss of the uplink and/or the downlink channel synchronization. After a while the UE will transit to a RRC_idle state and will need to re-access the network. The above described procedure involves however a time consuming operation, which includes inter alia: a setup of signaling connections to a radio access network (UTRAN) and a core network (CN); an authentication; and setting up new bearers. In addition, all data in buffers will be lost.
It is thus a problem that there is no fast recovery for a UE in case of a failed handover procedure as described above. A consequence of a failed handover procedure is that the end-user quality suffers substantially since the connection outage is in the order of several seconds. For real-time services, like VoIP, the actual experience for the user is a connection loss.
An additional drawback is that when a downlink connection HS-DSCH transport channel with a HS-DSCH serving cell is lost, a UE can potentially create an increased amount of inter-cell interference in cells outside of its Active Set. This is due to that the UE remains power-controlled from the cells in the Active Set and adjusts its power-levels only to the transmission power control (TPC) commands received from the cells in the Active Set. As there is no more means for the network to update this Active Set, the UE may move far into cells which it is not power-controlled from and create in these cells a considerable amount of interference.
In the international patent application WO 2006/000876, a method and apparatus are provided for defining terminal behaviour in a case where the terminal detects that it cannot receive the HS-DSCH from a serving cell reliably when a control channel is mapped to the HS-DSCH. The terminal is autonomously moved to another protocol state.